The present disclosure relates in general to optical system monitoring, and, more particularly, to a method and system for measuring optical scattering characteristics.
Optical fibers increasingly constitute the chief means for transmitting information through the world""s telecommunications network. Certain characteristics of an optical fiber can also be used to generate information rather than just transmit it. Specifically, the temperature of an optical fiber affects the amount and wavelength of light that will be scattered in response to a transmitted pulse. Careful measurements of scattered light can therefore be used to determine the temperature at points along an optical fiber. As another example, mechanical stresses on the fiber affect the amount of certain wavelengths of light that will be scattered in response to a transmitted pulse. Once again, measurements of scattered light can yield useful information.
Other optical systems also scatter light in correlation with characteristics of interest. For example, an air-filled region may scatter light in proportion to the density of pollutants or another constituent element of interest. Accurately measuring the extent to which certain wavelengths or ranges of wavelengths of light are scattered provides information about other characteristics of the system.
In a conventional method a time-limited pulse of light with an electromagnetic spectrum of average wavelength xcex is produced at an excitation source and sent through an optical fiber. When the excitation source is a laser, the electromagnetic spectrum is often very narrow and is referred to in shorthand as a single wavelength. As the pulse traverses the fiber, backward scattered light is produced. Three types of backward scattered light, among others, are of interest: Stokes light, anti-Stokes light, and Rayleigh light. Stokes and anti-Stokes light are collectively referred to as Raman light. Stokes light constitutes an electromagnetic spectrum having an average wavelength greater than xcex. Anti-Stokes light constitutes an electromagnetic spectrum having an average wavelength less than xcex. Rayleigh light has the same wavelength xcex as the excitation source. The width of the Stokes and anti-Stokes spectra, as measured by the difference in wavelength between the points of 50% intensity, is often much greater than the width of the time-limited pulse spectra and the Rayleigh light spectra, especially if that pulse is produced by a laser.
Some of the Rayleigh, Stokes, and anti-Stokes light travels to the end of the fiber at which the pulse was introduced, while some is scattered at an angle such that it is absorbed by the cladding of the fiber or escapes. The location from which the backward scattered light originated can be determined by the time between the introduction of the pulse and the receipt of the light. After a pulse is introduced into the fiber, backward scattered light is continuously received and time functions of the total intensity across the Stokes and anti-Stokes spectra can be determined. Under particular circumstances, the temperature of a point in the fiber has a known relationship to the ratio of the anti-Stokes light produced at that point to the Stokes light produced at that point. If, however, the intensity of the excitation per area of the fiber core is too high, non-linear distortions eliminate the temperature proportionality. Increasing the measurement accuracy of Stokes and anti-Stokes intensity as a function of time without introducing non-linear distortion, increases the accuracy of the resulting calculation of temperature as a function of position in the fiber.
U.S. Pat. No. 5,113,277 discloses a Fiber Optic Distributed Temperature Sensor System. The ""277 patent contemplates introducing a light pulse from a light source into a fiber. The scattered light is then divided by wavelength spectra with detectors positioned to receive the Stokes light and anti-Stokes light, respectively. The measurements made by the detectors are then introduced into an equation to determine the temperature at each measured distance.
The use of timed pulses of light to detect temperature or mechanical stress can require expensive components. For example, a light source that has sufficient power and produces light of a wavelength that has scattering characteristics allowing for measurements of scattering over a long distance of fiber can be very expensive. Additionally, the electronics necessary to convert the received intensity of back scattered radiation into a digital representation become more expensive as their processing speed increases. Increasing the spatial resolution of the temperature measurements using timed light pulses requires digital representations of back scattered radiation intensity for smaller periods of time. Such representations are only available with the use of faster, and consequently, more expensive electronics. Additionally, high power pulses can cause stimulated emission of Raman light. Such stimulated emission cannot be distinguished from backscattered radiation and renders calculations inaccurate.
The time pulse method disclosed in the ""277 patent also uses optical components to screen Rayleigh scattered light from the sensors. Analyzing the characteristics of Rayleigh scattered light can result in useful information indicating possible mechanical stresses in the optical fiber. This information is not available when the wavelengths comprising the Rayleigh scattering are blocked from the sensors.
A method and system of measuring optical scattering characteristics is disclosed. None of the advantages disclosed, by itself, is critical or necessary to the disclosure.
A system is disclosed for measuring optical scattering characteristics that includes a laser that produces an excitation signal. An optical fiber is coupled to the laser. At least a portion of the excitation signal enters the optical fiber as a coupled excitation signal with a continuous waveform and an amplitude modulated at variable frequencies. A first detector receives radiation backscattered from the coupled excitation signal by the optical fiber. In a more specific embodiment, the coupled excitation signal has a power less than 500 mW. In another more specific embodiment, the optical fiber is a single mode optical fiber.
A method is disclosed for measuring optical fiber characteristics that includes coupling a continuous wave laser excitation signal to an optical fiber. Radiation backscattered by the optical fiber in response to the coupled excitation signal is detected to produce a backscattered radiation signal. The backscattered radiation signal is mixed with the excitation signal to produce a mixed signal. The mixed signal is filtered to reduce the magnitude of frequencies other than conjugate mixing frequencies relative to the conjugate mixing frequencies. The filtered signal is digitized and the magnitude of backscattered radiation from a specific portion of the fiber is calculated based on the digitized signal. In a more specific embodiment, the temperature of a specific portion of the fiber is determined from the magnitude of the backscattered radiation.
It is a technical advantage of the disclosed methods and systems that backscattered radiation from an optical target receiving a variable frequency interrogation signal is detected.
It is also a technical advantage of the disclosed methods and systems that less expensive electronics can be used to monitor variable frequency backscattering.
Another technical advantage of the system and method disclosed is that lower cost excitation sources producing less power can be used to produce accurate results.
Another technical advantage of the system and method disclosed is that a lower power excitation signal can be coupled to a fiber to reduce non-linear distortion.
Another technical advantage of the system and method disclosed is that the temperature at a specific point of the optical target can be determined.
Another technical advantage of the system and method disclosed is that mechanical stresses of an optical fiber can be determined.
Another technical advantage of the system and method disclosed is that the particle density of an air-filled region can be determined.
Another technical advantage of the system and method disclosed is that the frequency difference between the detected reference signal and the detected backscattered radiation can be used to determine the origin of the backscattered radiation.
Another technical advantage of the system and method disclosed is that the backscattering characteristics of single mode fiber can be determined.
Other technical advantages of the present disclosure will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Various embodiments of the invention obtain only a subset of the advantages set forth. No one advantage is critical to the invention. For example, one embodiment of the present invention may only provide the advantage of detecting backscattered radiation, while other embodiments may provide several of the advantages.